An alloy rod having giant magnetostriction such as an amount of magnetostriction of, for example, at least 10.sup.-3 is now attracting the general attention as a material for elements of an electric audio converter, a vibrator, an actuator and the like, and industrialization thereof is leader way. The alloy rod having such large magnetostriction is usually manufactured by heat-treating a rod-shaped alloy material at a temperature slightly lower than the melting point thereof, or totally melting a granular or flaky alloy material and then solidifying the resultant melt of the alloy material into a rod shape.
An alloy comprising at least two rare earth metals including terbium (Tb) and dysprosium (Dy) and at least one transition metal is available as an alloy material for the alloy rod having such giant magnetostriction. As the above-mentioned alloy, an alloy having the following chemical composition is known in the product name of "Terfenol": EQU Tb.sub.X Dy.sub.Y Fe.sub.Z
where, X, Y and Z are ratios of the number of atoms, taking respectively the following values:
For the manufacture of an alloy rod having giant magnetorstiction, the following methods are known:
subjecting a rod-shaped alloy material having a chemical composition comprising Tb.sub.0.28 Dy.sub.0.72 Fe.sub.2 to a heat treatment in an inert gas atmosphere, which heat treatment comprises heating said alloy material to a temperature slightly lower than the melting point thereof by means of an annular heater stationarily arranged so as to surround said alloy material while moving said alloy material in the axial direction thereof, thereby manufacturing an alloy rod having giant magnetostriction (hereinafter referred to as the "prior art 1"). PA1 receiving a rod-shaped alloy material having a chemical composition comprising Tb.sub.X Dy.sub.1-X Fe.sub.1.5-2.0 (X being from 0.27 to 0.35) into a chamber made of quartz in an inert gas atmosphere; moving an annular high-frequency heating coil, arranged so as to surround said chamber, from the lower end toward the upper end of said chamber to heat said rod-shaped alloy material in said chamber in the circumferential direction thereof; continuously moving said heating coil from the lower end toward the upper end of said alloy material in the axial direction thereof to locally and sequentially melt said alloy material in the axial direction thereof; and then, locally and sequentially solidifying the resultant molten section of said alloy material in said chamber, thereby manufacturing an alloy rod having giant magnetostriction (hereinafter referred to as the "prior art 2"). PA1 supplying a granular or flaky alloy material, comprising at least two rare earth metals including terbium and dysprosium and at least one transition metal, into a crucible in an inert gas atmosphere placed in a vertical cylindrical heating furnace; totally melting said alloy material in said crucible in said heating furnace; and then, vertically moving any one of said crucible and said heating furnace to solidify and crystallize the resultant melt of said alloy material in said crucible at a lower portion of said heating furnace thereby manufacturing an alloy rod having giant magnetostriction (hereinafter referred to as the "prior art 3"). PA1 characterized in that: PA1 said inert gas atmosphere in said crucible is kept under a pressure within a range of from 0.2 to 10 atm.; and PA1 any one of said crucible and said heating furnace is moved at a speed within a range of from 0.1 to 5.0 mm/ minute in a temperature region of from 1,270.degree. to 1,180.degree. C. of said heating furnace, in which a temperature decreases at a temperature gradient within a range of from 10.degree. to 100.degree. C./ cm, to crystallize said melt of said alloy material in said crucible.
The above-mentioned prior art 1 has the following problems: A rod-shaped alloy material having a chemical composition comprising Tb.sub.X Dy.sub.Y Fe.sub.Z is brittle in general. Therefore, an alloy rod having giant magnetostriction manufactured by applying a heat treatment to the rod-shaped alloy material having such a chemical composition is also brittle and easily cracks. In addition, the heat treatment applied to the rod-shaped alloy material requires a long period of time, thus leading to a low manufacturing efficiency.
The above-mentioned prior art 2 has the following problems: When locally and sequentially melting the rod-shaped alloy material in the axial direction thereof and then locally and sequentially solidifying the resultant molten section of the alloy material, the molten section is held between the not yet melted alloy material and the melted and solidified alloy rod under the effect of surface tention of the molten section. However, the Tb.sub.X Dy.sub.Y Fe.sub.Z alloy in a molten state has only a small surface tension, with a high density. When a diameter of the alloy material is large, therefore, the molten section between the not yet melted alloy material and the melted and solidified alloy rod falls down in the form of drops, thus making it impossible to manufacture the alloy rod. According to the experience of the inventors, a diameter of the alloy rod capable of being stably manufactured in accordance with this method is 10 mm maximum even when adjusting the frequency and the output of the highfrequency heating coil for melting the alloy material. There is at present a demand for an alloy rod having giant magnetostriction, which has a large diameter of over 10 mm, for use as elements for an electric audio converter, a vibrator, an actuator and the like having a large output. However, such a large-diameter alloy rod having giant magnetostriction cannot be manufactured by the prior art 2.
The above-mentioned prior art 3 has the following problems: According to the prior art 3, it is possible to manufacture an alloy rod having a large diameter of over 10 mm. However, in order to manufacture an alloy rod having giant magnetostriction from an alloy material comprising at least two rare earth metals including terbium and dysprosium and at least one transition metal in accordance with the prior art 3, it is necessary to solidify and crystallize the melt of the alloy material in the crucible so as to achieve a single-crystal structure or a unidirectional-solidification structure consistent with the axial line thereof. However, it is not necessarily easy to solidify and crystallize the melt of the alloy material in the crucible so as to achieve a single-crystal structure or a unidirectional-solidification structure consistent with the axial line thereof. According to the prior art 3, therefore, it is difficult to stably manufacture an alloy rod having giant magnetostriction at a high efficiency.
Under such circumstances, there is a strong demand for the development of a method for manufacturing stably at a high efficiency an alloy rod having a large diameter of over 10 mm and giant magnetostriction without causing cracks, but such a method has not as yet been proposed.